The present invention generally relates to an improved apparatus for supporting a workpiece during a manufacturing process. The present invention more particularly relates to an improved wafer boat for holding at least one semiconductor wafer.
During manufacturing processes for producing integrated circuit devices on the surface of one or more semiconductor wafers, the wafers must be maintained in a predetermined orientation. The wafers are preferably maintained upright, in a non-touching orientation, in order to minimize the risk of contamination or damage to the wafers.
Many semiconductor manufacturing processes require extreme environmental conditions. For example, the wafer may be exposed to harsh chemicals such as hydrofluoric acid for etching layers on the surface of the wafer. Also, the wafer may be exposed to extremely high temperatures, as high as hundreds of degrees Celsius. Such high temperatures are used for driving implanted ions into the active layers of the wafer and in other processing steps. The apparatus for holding or supporting the wafers must withstand the extreme environmental conditions to which the wafers are exposed.
For holding or supporting semiconductor wafers under the most extreme conditions, a quartz wafer boat has been developed as the industry standard. Quartz has several features which make it amenable to this purpose. Quartz has a high melting point, so it is able to withstand the extreme temperatures used in wafer processing. Quartz is relatively impervious to the chemicals used in semiconductor processing. Quartz is not subject to outgassing, or the driving off of some of the impurities in the quartz itself during heating. These impurities may contaminate the wafers which the quartz boat is holding. Another material used for making wafer boats, having similar properties, is fused silica.
A quartz wafer boat is generally made by welding together a number of quartz members such as tubes or rods. The tubes may be hollow to reduce the weight and cost of the wafer boat. The tubes or rods are generally round in cross-section. Members are generally substantially transparent. A series of co-planar grooves is cut into some of the members. Semiconductor wafers are placed into the grooves, which maintain the wafers in a predetermined, non-touching generally upright orientation on the wafer boat.
It is desirable to uniquely identify a wafer boat. Identifying the wafer boat identifies the wafers it holds, which is important for tracking the progress of the wafers through the manufacturing process. Identifying the wafer boat is also important for correlating the performance or electrical characteristics of the finished integrated circuit devices on a wafer with processing conditions. If a statistically significant number of wafers from the same boat exhibit a similar characteristic, conclusions can be drawn about the processing experienced by the wafers held by that boat. Further conclusions can be drawn if wafers on different boats exhibit different characteristics.
However, because of the extreme processing conditions to which the wafers, and the wafer boat, are exposed, it has heretofore been difficult to reliably identify a wafer boat. Any means for identifying the wafer boat must withstand the same processing conditions as the wafer boat and still be legible after processing. Thus, paper or plastic or metal identifying tags would be inappropriate, as they would burn or melt at high temperatures. In addition, any means for identifying the wafer boat should exhibit the same desirable characteristics of the quartz used to fashion the wafer boat, such as high melting point and resistance to outgassing. Still further, because of the extreme processing conditions, any means for identifying the wafer boat must have chemical, mechanical and thermal properties similar to the quartz used to fashion the wafer boat. For example, the means for identifying the boat should have substantially the same thermal expansion properties as the quartz boat to prevent cracking, breaking or other damage during heating and cooling.
One technique currently used for identifying a wafer boat is marking one or more of the quartz members with a vibrating pencil or a laser. A code, including one or more letters or numbers, is etched or inscribed on the surface of the quartz member. Another technique involves welding a substrate to the quartz member to form identifying characters.
These prior art identification techniques have had several shortcomings, however. Since the quartz members are generally round in cross-section, marking the quartz with either a vibrating pencil or a laser is difficult. Welding is labor-intensive and greatly adds to the cost of the wafer boats, which may be discarded after only a few process runs. When letters or numbers are inscribed on the surface of the quartz, the letters or numbers are visible within only a narrow viewing angle. Inscribed letters and numbers generally cannot be read automatically, but must be read by a human operator, limiting the possibilities for process automation. All of these prior art marking techniques degrade as a result of processing, as the surface of the quartz becomes pitted and inscribed or welded figures become difficult or impossible to read.
Accordingly, there is a need in the art for a wafer boat identification system for identifying a wafer boat, for withstanding extreme processing and environmental conditions, and susceptible to automatic reading of the identifying code.